Figure 1.

Incubation at 19°C reversibly blocks MHC class II molecule maturation. B4-14 cells were pulsed for 20 min with [³⁵S]methionine and chased at 19°C for 60 min (1st through 7th lanes) or for 180 min (8th lane) and shifted to 37°C for the indicated times. Positive control cells (1st, 9th, and 10th lanes) were directly chased at 37°C for the indicated times. After cell lysis, I-A^b molecules were immunoprecipitated with the Y3P mAb. The samples were not boiled before SDS-PAGE analysis. Labeled class II molecules were not detected after the 20-min pulse (1st lane) because the Y3P mAb used for immunoprecipitation does not detect immature αβ dimmers complexed with intact Ii chain.